The present invention relates to an antenna arrangement comprising several base station antennas mechanically attached to each other. Such an arrangement is henceforth termed combined antenna and the antennas forming the combined antenna are called discrete antennas.
Cellular network operators often have licenses for more than one type of system, e.g. 2nd generation systems such as GSM or CDMA, 3rd generation cellular systems such as WCDMA, CDMA 1×EV-DO or TD-SCDMA, or 4th generation cellular systems such as LTE or IEEE16-m (WiMAX). Usually, a specific frequency band is allocated for each cellular system type, but in some cases different cellular systems can operate on the same band. For cost and other reasons, operators tend to co-locate the different cellular systems on the same site. In most cases, it is not desirable to use the same antenna for different systems, so the operator will often have two or more antennas pointing in the same direction, one for each cellular system. Also, if the operator has a license for two or more sub-bands within the same cellular frequency band, he may prefer to use two antennas rather than combining the carriers before feeding them to a common antenna as this will eliminate the combining losses. There are several disadvantages associated with having a large number of antennas at the same site: visual impact, higher wind load, higher cost of installation, higher rental cost for the site, etc. Therefore, it is often preferred to combine several discrete antennas into one unit sharing a common radome for environmental protection as this will be perceived as one slightly larger antenna. The antennas being combined together can be antennas made for the same frequency band, or antennas made for different frequency bands.
Combined antennas already exist and are widely deployed today. Typically, two or more antennas are being mechanically attached to each other with a common radome. Today most antennas have dual polarisation, with one polarisation being oriented +45 degrees relative to the antenna vertical axis, and the other polarisation oriented −45 degrees relative to the same axis, but the phenomena described below is likely to occur also with single polarisation antennas. A well-designed antenna will have a main lobe which, in the azimuth plane, points in a direction that is perpendicular to the antenna reflector and that is symmetrical with respect to an axis perpendicular to the reflector, but when antennas are placed close to each other, scattering and diffraction phenomena will occur because of the neighbouring antenna, and these phenomena may have a negative effect on the antenna radiation pattern, especially in the azimuth plane; the azimuth lobe width may increase or decrease, or the main lobe may point in a direction that is not perpendicular to the reflector in the azimuth plane, or the main lobe may become non-symmetrical or there may be a combination of the effects described above. The antenna radiation pattern in the elevation plane is less likely to be affected by the neighbouring antenna.
The antenna azimuth lobe width is important because it affects coverage and antenna gain. It is often important to have as high gain as possible in an antenna as this increases the size of the cell, and increases the capacity of the system. A narrower lobe will increase the gain, but may lead to reduced coverage. A wider lobe will reduce the gain, and may lead to interference problems as signal from one sector may leak into the neighbouring sector. When using combined antennas, it is usually assumed that the antenna lobes of two combined antennas point in the same direction, but if the two antennas lobes point in different directions due to scattering and diffraction phenomena, this will result in deteriorated coverage or increased interference in the network.
The object of this invention is therefore to provide means to reduce the effects of scattering and diffraction in a combined antenna. This object is obtained by arranging one or more conducting elements in the form of wires or strips between the discrete antennas arranged alongside each other in a combined antenna.
This invention relates to a combined base station antenna comprising two or more discrete antennas. The discrete antennas can be designed for the same frequency band, or different frequency bands. The antennas can have fixed or variable tilt and both types of tilt can be used in the same combined antenna. A typical non-limiting realisation of such an antenna is shown in FIG. 1 where two identical dual polarised antennas have been combined. The gain and azimuth lobe width of one discrete antenna alone behaves well over frequency as can be seen in FIG. 3 showing the gain for both polarisations. But when two discrete antennas are combined mechanically together, the azimuth lobe width increases from 67 to 73 degrees at the lower frequencies, and resulting in loss of gain in the order of 0.5 dB as can be seen in FIG. 4, a quite significant loss in performance.
One or more conducting elements in the form of wires or strips arranged in parallel with the antenna longitudinal direction, between two discrete antennas arranged alongside each other, will act as a reflector, and will reduce or almost eliminate the deterioration of the antenna gain caused by the neighbouring antenna as can be seen in FIG. 5. Conducting elements such as wires or strips have also been described to be used in base station antennas, e.g. in U.S. Pat. No. 5,952,983, but the use has then only been restricted to the reduction of cross-coupling between the two polarisations in a discrete dual-polarised antenna. The mode of operation is different, the wires are typically placed in a direction perpendicular to antenna longitudinal axis, and the length of the wires is typically in the order of one half wave-length, whereas in the present invention, the wire can run along the whole length of the antenna along the antenna longitudinal axis. In US 2006/0038376 A1, a suspended wire is used to reduce coupling between two antennas of a mobile phone that share a common ground plane; a wire with a length typically in the order of one half wave-length is placed above the common ground plane.
The object of the present invention is to reduce the effects of scattering and diffraction, and uses conductive elements such as wires or strips oriented in a direction parallel with the antennas longitudinal axis and having lengths that significantly exceed one half wave-length.